1. Field of the Invention
This invention relates to fans, and more particularly, to measuring the rotational speed of a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include at least one fan to aid in system cooling. The increased airflow provided by fans aids in eliminating waste heat that may otherwise build up and adversely affect system operation.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so.
Fans often include a tachometer output that provides a signal indicative of the current speed of the fan. The tachometer signal may be used to determine whether the fan is operating properly. Often, fans used for CPU and/or computer system cooling have a three-wire interface with wires for power, ground, and the tachometer signal. Fan drive systems often use a PWM (pulse width modulator) to drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the speed of the fan. PWMs are useful because they provide a digital control for the pulse width of a signal. The fan is turned on for the duration of the pulse and turned off between pulses. The duty cycle of the pulse train currently being provided to the fan determines the fan's speed.
One problem that results from using PWMs to drive fan circuits is that the fan is not powered for a fixed amount of time between each pulse. During this time, the tachometer circuitry associated with the fan does not receive power. As a result, the tachometer signal output by the fan may not accurately represent the current fan speed during the time between pulses. Similarly, when the fan is turned off, the tachometer signal does not indicate the speed of the fan as the fan spins down. One technique that is currently used to measure fan speed in these situations involves using an analog filtering system to measure the back EMF (Electromotive Force) inserted into the fan tachometer signal by the rotating fan. However, it is desirable to be able to accurately detect fan speed using a digital technique.